Undercut leading edge for compressor blades and related method

ABSTRACT

A compressor blade includes an airfoil portion having a leading edge, a radially inner attachment portion, and a platform between the airfoil portion and the attachment portion, wherein material is removed from the attachment portion to form an undercut at a front face thereof to thereby provide an overhang radially inward of the platform and leading edge of the airfoil portion, the undercut defined by a narrow entry slot opening into a rearward transverse groove.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to compressor blades, and specifically, to the provision for an undercut located radially inward of the leading edge of the airfoil portion of the blade.

[0002] In large gas turbines used for generating electricity, power companies regularly water wash the machines as soon as any performance degradation is noticed. The water wash is sprayed into the machine at the compressor end, near the hub, and the fluid is flung out into the flow path, cleaning the compressor blades. As a result of this water wash, water droplets impact the first stage blades causing significant erosion along their leading edges, especially at the hub of the airfoil, i.e., where the airfoil meets the platform. This leading edge erosion reduces the high cycle fatigue capability of the material and, in the presence of vibratory excitation, may lead to blade failure.

[0003] Accordingly, there is a need to create a more erosion tolerant blade by lowering the mean and vibratory stress levels at the leading edge of the airfoil portion of the blade.

BRIEF DESCRIPTION OF THE INVENTION

[0004] In accordance with an exemplary embodiment of this invention, attachment material (i.e., material in the root portion of the blade used to secure the blade to the compressor rotor or wheel) directly below, i.e., radially inward of, the airfoil leading edge is removed. This creates an undercut below the hub of the blade, i.e., below the blade platform and the leading edge of the airfoil. The void or space created by the material removed may be filled, if necessary, with a discrete acoustic damper of substantially the same shape as the void. The undercut arrangement effectively unloads the leading edge of the airfoil, thereby reducing the local mean and vibratory stresses along the leading edge and thus allows the blade to sustain considerably more damage from water washing and water droplet impact.

[0005] In the exemplary embodiment, the undercut is shaped to include a narrow transverse entry slot that opens into a rearward transverse and generally round groove that extends radially inwardly of the base of the slot. In other words, the groove is deeper than the slot in a radial direction, giving the appearance of a stylized, sideways “P.” Describing the slot and groove as “transverse” refers to the cross-wise orientation of the slot and groove across the width of the shank portion. Reference to a “radial direction” is with respect to a bucket mounted on the periphery of a turbine rotor wheel.

[0006] Accordingly, in its broader aspects, the present invention relates to a compressor blade comprising an airfoil portion having a leading edge, a radially inner attachment portion, and a platform between the airfoil portion and the attachment portion, wherein material is removed from the attachment portion to form an undercut at a front face thereof to thereby provide an overhang radially inward of the platform and leading edge of the airfoil portion, the undercut defined by a narrow transverse entry slot opening into a rearward transverse groove.

[0007] In another aspect, the invention relates to a compressor blade comprising an airfoil portion having a leading edge, a radially inner attachment portion, and a platform between the airfoil portion and the attachment portion, wherein material is removed from the attachment portion to form an undercut at a front face thereof comprising a narrow transverse entry slot opening into a rearward transverse groove to thereby provide an overhang radially inward of the platform and leading edge of the airfoil portion; wherein, when assembled on a compressor wheel, a void space created by the undercut is substantially filled by an acoustic damper.

[0008] In still another aspect, the invention relates to a method of unloading a leading edge of an airfoil portion of a compressor blade comprising: a) providing a blade having an airfoil portion with a leading edge, a platform, and an attachment portion adapted to secure the blade to a compressor wheel; and b) removing material from the attachment portion to create an undercut radially inward of the leading edge of the airfoil portion, the undercut defined by a narrow transverse entry slot opening into a rearward transverse groove.

[0009] The invention will now be described in conjunction with the drawing figures identified below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a perspective view of a compressor blade with an undercut leading edge, and also showing an associated acoustic damper; and

[0011]FIG. 2 is an enlarged detail taken from FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0012] With reference to FIGS. 1 and 2, a compressor blade 10 includes an airfoil portion (or simply, airfoil) 12, a platform 14; and an attachment or root portion 16 that typically is formed with a dovetail configuration that enables the blade to be loaded into a complimentary groove on the compressor wheel or rotor (not shown). Material has been removed from the forward end of the attachment portion 16, thereby creating an undercut 18 radially inward of the platform 14 (relative to the center of a rotor wheel on which the blade is mounted) and the leading edge 20 of the airfoil 12. The undercut is shaped to include a narrow transverse entry slot 22 that opens into a rearward transverse groove 24 (FIG. 2) that extends radially inwardly of the base 26 of the slot 22. In other words, the groove 24 is deeper than the slot 22, giving the appearance of a stylized, sideways “P.” The void space created by the undercut may be filled by an acoustic damper 28. It will be understood that the damper 28 may be composed of material of the same composition as the blade, or other material such as nylon or other suitable high strength plastics. The damper preferably substantially matches the configuration of the void space created by the undercut 18. Thus, the damper 28 has a relatively thin flange portion 30 that connects to an rearward portion 32. It will be appreciated that the flange portion 30 will fit within the narrow entry slot 22 and the round rearward portion 32 will fit in the groove 24. The configuration as described is significant in that the narrow entry slot 22 reduces windage losses while the round groove 24 serves to reduce the stress concentrations adjacent the leading edge 20 of the airfoil portion 12.

[0013] In the exemplary embodiment, the groove 24 may have a diameter of about {fraction (1/2)} inch and the slot 22 may have a height of about {fraction (1/8)} inch. These dimensions, as well as the length of the slot 22 (as measured from the forward edge of the blade to the groove 24) may vary in accordance with blade size. In any event, the dimensions must be sufficient to offload the leading edge 20 of the airfoil 12, but must not be so large as to negatively effect the loading of the blade 10 as a whole.

[0014] By creating the undercut 18 to effectively unload the leading edge 20 of the airfoil 12, the airfoil can sustain considerably more damage from water washing without exceeding the material capability. Specifically, the erosion process creates small cracks in the leading edge of the airfoil 12. When the crack length exceeds a propagation threshold value, the blade fails. In a conventional blade with a fully supported leading edge, this edge becomes the life limiting location. Undercutting the blade in accordance with this invention moves the life limiting location to an area approximately 1 inch aft of the leading edge, at the root, on the pressure side of the airfoil. This location is not subjected to the constant water droplet pounding of the waterwash. Consequently, the cracks created by the water washing will no longer propagate and endanger the machine. The leading edge 20 of the airfoil 12 will simply erode with time.

[0015] With the above arrangement, a more erosion tolerant blade is achieved by lowering the mean and vibratory stress at the leading edge of the blade.

[0016] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. A compressor blade comprising an airfoil portion having a leading edge, a radially inner attachment portion, and a platform between the airfoil portion and the attachment portion, wherein material is removed from the attachment portion to form an undercut at a front face thereof to thereby provide an overhang radially inward of the platform and leading edge of the airfoil portion, the undercut defined by a narrow transverse entry slot opening into a rearward transverse groove.
 2. The compressor blade of claim 1 wherein, when assembled on a compressor wheel, a void created by the undercut is filled by an acoustic damper having substantially the same shape as the void.
 3. The compressor blade of claim 2 wherein said acoustic damper is constructed of a high strength plastic material.
 4. The compressor blade of claim 3 wherein said acoustic damper is constructed of nylon.
 5. The compressor blade of claim 1 wherein said groove is round and has a diameter of about 0.5 inch.
 6. The compressor blade of claim 1 wherein the undercut extends in an axial direction at least to the leading edge of the airfoil portion.
 7. A compressor blade comprising an airfoil portion having a leading edge, a radially inner attachment portion, and a platform between the airfoil portion and the attachment portion, wherein material is removed from the attachment portion to form an undercut in a front face thereof at least and including a transverse, substantially round groove rearward of said front face to thereby provide an overhang radially inward of the platform and leading edge of the airfoil portion; wherein, when assembled on a compressor wheel, a void space created by the undercut is substantially filled by an acoustic damper.
 8. The compressor blade of claim 7 wherein said acoustic damper is constructed of a high strength plastic material.
 9. The compressor blade of claim 7 wherein said acoustic damper is constructed of nylon.
 10. The compressor blade of claim 7 wherein a narrow transverse entry slot opens into said transverse groove, said groove having a diameter of about 0.5 inch.
 11. A method of unloading a leading edge of an airfoil portion of a compressor blade comprising: a) providing the blade having the airfoil portion with the leading edge, a platform, and an attachment portion adapted to secure the blade to a compressor wheel; and b) removing material from the attachment portion to create an undercut radially inward of the leading edge of the airfoil portion, and defined by a narrow transverse entry slot opening into a rearward transverse groove.
 12. The method of claim 11 wherein, when assembled on the compressor wheel, a void created by the undercut is filled by an acoustic damper having substantially the same shape as the void.
 13. The method of claim 12 wherein said acoustic damper comprises a high strength plastic material.
 14. The method of claim 12 wherein said acoustic damper comprises nylon.
 15. The method of claim 11 wherein the undercut extends in an axial direction at least to the leading edge of the airfoil portion.
 16. The method of claim 11 wherein said rearward transverse groove is substantially round in shape.
 17. The method of claim 16 wherein said groove has a diameter of about {fraction (1/2)} inch. 